fix docs langgraph persistence checkpoints md (#1766)

## Overview
Fix typo in import statement: corrected incorrect `MemoryServer`
reference to the valid `MemorySaver` export from `@langchain/langgraph`,
aligning the code example with the actual library API (eliminates import
errors for end users).

## Related issues/PRs
- GitHub issue: N/A
- Feature PR: N/A

<!-- For LangChain employees, if applicable: -->
- Linear issue: N/A
- Slack thread: N/A

## Checklist
- [x] I have read the [contributing guidelines](README.md)
- [x] I have tested my changes locally using `docs dev`
- [x] All code examples have been tested and work correctly
- [x] I have used **root relative** paths for internal links
- [x] I have updated navigation in `src/docs.json` if needed

## Additional notes
The code example incorrectly referenced `MemoryServer` (a non-existent
export from `@langchain/langgraph`), which would cause import errors for
users implementing the example. This change corrects the reference to
`MemorySaver`—the official, valid export for persistent memory
management in LangGraph. No other functional or structural changes are
included; this is a pure typo fix to ensure the example is accurate and
runnable.

Co-authored-by: Lauren Hirata Singh <lauren@langchain.dev>

Author: sh-gongwenlong

src/oss/langgraph/persistence.mdx

@@ -125,7 +125,7 @@ await graph.invoke({ foo: "", bar: [] }, config);
 
 :::js
 ```typescript
-import { StateGraph, START, END, MemoryServer } from "@langchain/langgraph";
+import { StateGraph, START, END, MemorySaver } from "@langchain/langgraph";
 import { registry } from "@langchain/langgraph/zod";
 import * as z from "zod";